This invention relates generally to a xerographic copying device and, more particularly to an optical system which scans a document lying on an object plane by means of a pivoting mirror and projects the reflected document image onto a curved photosensitive surface.
Copying devices which utilize a fixed light source and pivoting mirror to scan a document on a curved platen are known in the art, e.g. the Xerox 2400 and 3600 machines. It is desirable to copy documents by placing them on a flat platen while retaining the scanning simplicity of a pivoting mirror arrangement. However, this has heretofore not been possible due to the basic problem of compensating for the "field tilt". Field tilt is the deviation of the image plane from parallelism to the ideal photoreceptor plane during scan. With use of a flat platen, a keystoning distortion effect also occurs because points on the image plane will vary in focus because of nonuniform magnification ratio or change in the ratio of the object and image conjugate distances during scan.
In one aspect of the present invention, there is provided an optical system which scans a document on a flat platen by means of a pivoting mirror and projects an image of said document onto a curved photoreceptor surface, the imaging being accomplished without the attendant problems of field tilt or keystoning distortion.
According to another feature of the invention, the scanned image is projected onto a moving photoreceptor at a rate faster than the rate of movement of the photoreceptor and in the direction opposite such movement. This technique, which can be termed as precessing the image, is generally known in the art. U.S. Pat. No. 3,454,335 (Caldwell) assigned to the same assignee as the present invention, discloses a method of projecting images on microfilm cards which are moved past a stationary lens and mirror system onto a drum photoreceptor. The drum and microfilm are moved at the same speed but the image is precessed (i.e. moved in a direction opposite the drum motion) onto the drum surface by means of a slit aperture. This combined slit and drum rotation exposes the drum at a speed (rate) that is greater than the surface speed of the drum. This scanning system demonstrates two of the advantages gained by image precession: the process speed of the system can be set at a lower speed for equivalent copy rate (copies per minute) and the gap ordinarily present between images (due to the return time of the scan optics) can be reduced or eliminated if desired. Scan return velocities for a given copy rate can also be minimized for a given copy rate, thus reducing scan accelerations, forces and vibrations. A problem with such a system, however, is that defocusing errors may result in developed images unacceptable for some systems. The errors are introduced because the image reflected from the drum mirror to the slit does not maintain perpendicularity to a tangential line at the drum surface.
This defocusing problem is addressed in U.S. Pat. No. 4,232,960 (Glab) assigned to the same assignee as the present invention. Glab solves the problem of field tilt in his particular scan system, by using linear and rotational motions of optical elements located near the drum surface to scan the image onto the drum at the angle corresponding to the angle of reflection of the image from the platen.
Another prior art device which utilizes precession onto a drum photoreceptor is disclosed in U.S. Pat. No. 3,650,621 (Lewis) also assigned to the same assignee as the present invention. Lewis discloses a system wherein an imaging device is moved in an arcuate path which is in a direction opposite to drum rotation to scan document on a curved platen onto the drum.
Another optical system for precessing an image onto a drum type photoreceptor is disclosed in copending U.S. Application Ser. No. 190,110 filed on Sept. 24, 1980 and assigned to the same assignee as the present application. In this copending application a dual rate scanning mirror is driven at a speed greater then the process speed of the drum photoreceptor. A drum reflector is adapted to reflect the image onto the drum surface in a direction opposite the drum rotation.
The present invention therefore is further directed to a scanning system which scans a document on a flat platen with a pivoting mirror arrangement while simultaneously precessing an image onto a curved photoreceptor without attendant defocusing problems. The invention is realized in a system which includes a projection lens having an object plane for supporting an object to be copied; a projection lens having a rotational and translational motion; a pivoting mirror for scanning said object and reflecting said light image towards said projection lens; an aperture slit located adjacent the curvilinear surface, the aperture slit moving in a direction opposite the direction of movement of the curvilinear surface; means for maintaining constant conjugate during scan; and means for synchronizing the motions of said lens, mirror and aperture slit so as to maintain parallel object, lens and image planes during the entire scan cycle; whereby said lens rotates simultaneously with said reflector and slit motion to continually maintain said projected image at the photosensitive surface at the same angle of incidence as the angle of the principal scanning ray at the object plane.